Peroooooo
"The whole discussion of slower weapons for poisons is based on this: Following your weapon speed and the proc per minutes of your poison, an average proc chance is calculated that will be applied every swing. You may very well have way more/less than your ppm procs in a minute, but on average it will be the same. The point now lies in the fact that every time your weapon hits the poison has a chance to proc. On sinister strikes, mutilates etc. the poison also has a chance to proc, and it uses this chance derived from your weapon speed. Using a slow weapon gets your average proc chance up, thus you proc it more often with your special attacks. The difference in damage is rather minimal though, so people often just follow the "slow MH fast OH" because they could always put crippling/deadly/mind-numbing on their OH without much reduced effect."
Poisons - WoWWiki - Your guide to the World of Warcraft
Igual si sos combat swords la OH rápida sirve para procs de Sword Spec (Hack & Slash) y Combat Potency.